Internal combustion engines often emit harmful oxides of nitrogen (“NOx”) during operation. These oxides form when nitrogen and oxygen, both of which are present in the air used for combustion, combine within the main combustion chambers. Typically, the level of NOx formed increases as the peak combustion temperatures within the combustion chambers increase. As such, minimizing the peak combustion temperatures within the main combustion chambers generally reduces the emission of NOx.
For this reason, leaner gas or leaner gaseous fuel mixtures are used for reducing the peak combustion temperatures in the main combustion chambers, thus reducing the amount of harmful NOx emitted. A lean gas or gaseous fuel mixture has a relatively large air-to-fuel ratio when compared to a gas mixture having a stoichiometric air-to-fuel ratio. Accordingly, using more air in the fuel mixture may advantageously lower NOx emissions.
Unfortunately, using a lean air-to-fuel ratio may result in incomplete combustion within the main combustion chamber, especially in engines with large bores, due to the relatively slow rate of flame propagation from a single point ignition source, such as a spark plug. Furthermore, turbulence within the main combustion chamber may extinguish the ignition flame before the lean gas or gaseous fuel combusts. If the ignition flame extinguishes prior to complete combustion, the power output of the engine reduces and the amount of uncombusted fuel emitted increases.
To minimize the occurrence of incomplete combustion, some internal combustion engines incorporate a precombustion chamber, or prechamber. These prechambers may be in fluid communication with the main combustion chambers of the engine via small orifices. Either enriched or non-enriched fuel may be advanced in these prechambers. Ignition of the fuel within the prechamber creates a front of burning fuel that is jetted through the orifices and into the main combustion chamber, thus igniting the lean fuel within the main combustion chamber. The front of burning fuel jetting out of the orifices is generally sufficient to cause complete combustion of the lean fuel within the main combustion chamber.
These prechambers, however, do not have adjustable volumes that may vary as a function of different engine operating conditions.
As engine load changes, it is desirable to vary the volume of the prechamber to control momentum formation and gas jet penetration during combustion. Varying the volume of the prechamber results in more repeatable combustion for lean fuel mixtures, mitigates engine knock, and provides for improved ignition response.
Additionally, it is also desirable to vary the volume of the prechamber based on other engine operating conditions in order to improve engine performance and decrease harmful emissions.